Having worked with, interviewed, and dissected several tieflings and aasimars over my long career with the Arcane Research Society, I have found that most of them are actually quite biologically similar to the human parent. In fact, many are much less than half outsider, the celestial or fiendish infusion to their lineages often tracing back several generations of seemingly normal human specimens, resulting suddenly with the very dominant outsider traits in the tiefling or aasimar. Such beings are merely "plane touched", showing little outward sign of extraplanar heritage.
That said, such beings aremortal, with souls, and I have personally raised such creatures as undead without complication. In earlier times, before taxonomists developed the "native outsider" moniker, such creatures were classified under their original type (humanoid), reserving the "outsider' classification for true halfbloods like cambions and alu-fiends. The debate and speculation continues.
My hypothesis is that tieflings and aasimar would in fact be changed by greater undead like morghs, owing to their largely human content. I move that the "outsider, native" definition be expanded to include that such creatures- or perhaps better to add "human blood" as a racial trait for aasimar and tieflings (as with half-orcs and half-elves) if they remain under the outsider heading so that their humanoid heritage is accounted for. More experiments are needed.
